1. Field of the Invention
The present invention relates to a sensor apparatus for measuring an object in a sample using light. More particularly it relates to a sensor apparatus for detecting or measuring a specific substance by using the interaction between light and a surface plasmon wave caused by the total-reflection of light on a metal thin film provided on a light-transmitting medium. The typical sensor apparatus is a nucleic-acid detecting device, in which a vertical cavity surface emitting laser (VCSEL) and a sensor array, such as an array of charge-coupled devices (CCDs), are arranged on a common substrate, and an optical system composed of integrally formed substrate, light-transmitting medium and metal thin film is employed.
2. Related Background Art
Conventionally, oxidation-reduction reaction of a measurement object, color reaction of a measurement object with color reagent, and the like have been used in chemical sensors for measuring sample concentrations. In those cases, when a highly-sensitive, highly-selective sensor is needed, it is preferable that the measurement object is used as a substrate and a biosubstance with a strong affinity for the substrate, such as an antibody for an antigen, is used for the substrate. Where the measurement object is nucleic acid, then a so-called probe nucleic acid can be preferably used. In this probe nucleic acid, a portion of a base arrangement in the nucleic acid is replaced by a complementary base arrangement.
Recently, a highly-sensitive method has been proposed to optically measure a change in the dielectric constant which accompanies a biochemical reaction (see Japanese Patent Application Laid-Open No. 61(1986)-292045). In this method, the interaction between light and surface plasmon wave is used. The surface plasmon wave is generated under a total-reflection condition of light on a metal thin film provided on a light-transmitting medium. Its principle of measurement is as follows.
FIG. 6 illustrates the structure of the above-discussed prior art measuring apparatus. In FIG. 6, light emerging from a light source 31 enters a prism 32 (a light-transmitting medium), is reflected at a reflective surface of the prism 32, and is detected by a photodetector 33. A spacer layer 34 of a buffer medium, a metal film 35 and an organic material layer 36 (an insulator) are serially desposited on the reflective surface of the prism 32. A sample fluid 37 of a measurement object is in contact with an external surface of the organic material layer 36.
A surface plasmon wave is defined herein as an electromagnetic wave generated at the interface between a metal and an insulator. This wave can be optically induced when the resonance condition determined by refractive index (i.e., dielectric constant) in the vicinity of the interface between the metal and the insulator and its thickness is satisfied. Initially, p-polarized light is caused to impinge on the light-transmitting medium with the metal thin film thereon such that a total reflection of the light occurs at the metal thin film. Then, an evanescent wave occurs with a wave number depending on the incident angle of light at the interface between the metal thin film and the light-transmitting medium. On the other hand, the surface plasmon wave is generated on an outer surface (a surface in contact with the insulator) of the metal thin film due to a tunneling effect of light. The surface plasmon resonance occurs when wave numbers of the evanescent wave and the surface plasmon wave respectively created on both faces of the metal thin film are coincident with each other. At this time, part of energy of the incident light is used to induce energy of the surface plasmon wave.
The intensity of light reflected at the metal thin film is equal to a difference between the intensity of the incident light and the light intensity lost by the excitation of the surface plasmon wave, based on the energy conservation law. Therefore, the surface plasmon resonance can be measured by measuring the incident-angle dependency of the intensity of the reflected light. The resonance condition is determined from the wavelength of incident light, its incident angle, complex dielectric constants of light-transmitting medium and metal thin film, complex dielectric constant of a sensor""s sensitive film provided on the metal thin film, and so forth. When the complex dielectric constant varies due to the biochemical reaction in the sensitive film, the resonance condition is changed. Hence, under the condition of a constant wavelength, the light incident angle for causing the surface plasmon resonance is varied. When this variation of the light incident angle is detected, the substrate concentration of the biochemical reaction, i.e., concentration of the measurement object, can be obtained.
Since the surface plasmon wave is generated in a region within about several hundred nanometers on the metal thin film, the biochemical reaction between substrate and biosubstance causing the change in the dielectric constant must be effected in this region. Therefore, a very thin film will suffice to form the sensitive film with the biosubstance fixed thereon. Further, only the neighborhood of the metal thin film can be measured in the surface plasmon resonance, so even a colored sample and a suspended sample can be measured without the influences of the color or suspension.
Hitherto, a detecting sensor of an antigen of protein, and the like have been developed using the surface plasmon resonance (for example, BIAcore by Phalmasia Co.). In this sensor, an organic thin film as the sensitive film is provided on the metal film on which the surface plasmon resonance occurs, and an antibody is fixed in the organic thin film. When the fixed antibody is selectively bonded to the antigen in the measurement object, the dielectric constant of the organic thin film is slightly changed. This change can be measured from a change in the resonant angle. This principle can also be used in a nucleic-acid sensor and the like, in which an organic thin film as the sensitive film is provided on the metal film on which the surface plasmon resonance occurs, and a nucleic acid or the like is fixed in the organic thin film. When the fixed target nucleic acid or probe nucleic acid is selectively bonded to probe nucleic acid or target nucleic acid in the measurement object, the dielectric constant of the organic thin film is slightly changed and this change can be measured from a change in the resonant angle.
Such a measuring apparatus using the surface plasmon resonance is disclosed in Japanese Patent Application Laid-Open Nos. 5(1993)-18890, 6(1994)-58873, 6(1994)-167443, 6(1994)-265336, 7(1995)-174693, xe2x80x9cSensors and Actuators B329 (1995) pp. 268-273xe2x80x9d, or xe2x80x9cSensors and Actuators B32 (1996) pp. 149-155xe2x80x9d, for example. In those apparatuses, a metal thin film is formed on a prism, and the surface plasmon resonance created by incidence light from outside of the prism is measured by a detector disposed on the outside of the prism. In those apparatuses, the incident angle of light incident on the metal thin film needs to be varied to measure a change in the resonant angle. Hence, the apparatus becomes relatively large including light source, prism, detector, movable device, and so forth. Accordingly, a sensor apparatus with a large elasticity is difficult to fabricate based on such a construction.
Further, the metal thin film for creating the surface plasmon resonance can achieve a sufficiently exact measurement with a very small area. Therefore, there have also been proposed sensor-type apparatuses in which only the measuring portion is shaped into a minute configuration. For example, xe2x80x9cSensors and Actuators B34 (1996) pp. 328-333xe2x80x9d proposed a sensor using an optical fiber. Since the group velocity of light propagated through an optical fiber is determined from its wavelength, incident and reflection angles of light totally reflected at the interface between the core and the cladding of the fiber are dependent on the wavelength of light and characteristics of the fiber.
Here, a portion of the cladding in the optical fiber is removed, and a metal thin film is deposited on the surface of the cladded portion. When the resonance occurs between an evanescent wave generated during the total reflection at the interface between core and metal thin film and a surface plasmon wave on the metal thin film at a resonant wavelength of various wavelengths, light at its resonant wavelength attenuates. Therefore, when white light is inputted into the optical fiber and the wavelength dispersion of light transmitted through the core and the metal thin film is detected, the attenuation of the light intensity in a wavelength range of the surface plasmon resonance can be measured. Those methods drastically increase a practicable potential of the sensor using the surface plasmon resonance. Those methods, however, require a strict optical positioning of the coupling between light source and optical fiber, the coupling between optical fiber and optical detector, and so forth.
A similar sensing technique using the optical fiber is further disclosed in Published European Patent Application No. 0282009. This technique is directed to an optical fiber sensor using a change in refractive index resulting from the interaction with hydrocarbons. The operating principle is based on a change in the refractive index of the cladded material caused by the presence of hydrocarbon.
Further, there has been proposed a device which includes no driving unit for changing the incident angle of light and in which a prism and an optical detector are integrally arranged. Japanese Patent Application Laid-Open No. 7(1995)-225185 discloses a sensor apparatus in which light waveguide, waveguide-type lens and CCD detector are arranged on a glass substrate, for example. Light from a semiconductor laser is inputted into this sensor, and the surface plasmon resonance is measured. The sensor is advantageous in that a relative positional relation between respective optical elements need not be adjusted after the fabrication of the sensor.
Furthermore, xe2x80x9cSensors and Actuators B35-36 (1996) pp. 212-216xe2x80x9d proposes a sensor apparatus in which a light emitting diode and a photodiode array are integrally arranged. In this sensor apparatus, all optical elements needed for the surface plasmon resonance measurement are packed in a single package, and a sensitive film is deposited on a metal thin film formed on the package to achieve the function of the sensor apparatus. In the sensor apparatus, a wide width of the incident angle of light incident on the metal thin film is obtained by using a wide expansion of light emitting from the light emitting diode, and the light intensity corresponding to each incident angle can be detected by the photodiode array. The sensor apparatus needs no adjustment of the optical arrangement for measurement of the surface plasmon resonance, and the sensor""s function is established by fixing biosubstance to the metal thin film. In this sensor apparatus, however, a sensor is needed for each measurement object, and the sensor apparatus is hence unsuitable for many-component sensing.
As described above, the chemical sensor using the surface plasmon resonance measures the change in the intensity of reflected light during the total reflection which depends on the incident angle. Accordingly, for the purpose of highly-precise measurement, an appropriate relative position between light source, p-polarizer, lens, light-transmitting medium and photodetector must be strictly established. Further, in the method for measuring the reflected light while changing the incident angle of light, respective optical elements must be moved with a high positional precision. To dispose those optical elements, highly-rigid material must be used to fix them, precision is required to mount and drive them, and the size of the apparatus inevitably increases since control systems for driving and so forth must be used.
Further, in the method for measuring the surface plasmon resonance at the core portion of the optical fiber, a versatile sensor can be obtained with high elasticity. However, problems occur in that: couplings of light source and photodetector to the optical fiber are needed; a reference portion is needed to cope with external influeneces of temperature and so forth; a sensor portion is needed for each measurement object in a many-component simultaneous measurement; and a large number of spectroscopes and photodetectors are needed because of the measurement of the surface plasmon resonance using wavelength dispersion. Thus, productivity and versatility of the sensor are reduced.
On the other hand, in the surface plasmon resonance sensor apparatus provided with an integrated arrangement of light source, polarizer, photodetector and light-transmitting medium, no strict positioning of the optical elements is needed and no driving unit is needed. Accordingly, it is possible with this integrated arrangement to achieve a small sensor size and stability in sensor response. However, when a light emitting diode is used as the light source, since light emerging from a radiation point of the diode expands in a conical form, only a single-component measurement can be performed even when the above-discussed photodiode array is used. Since, however, a variety of components are typically present in a sample, there is a great unfulfilled need for their simultaneous measurement. Further, where the concentration of the measurement object in the sample ranges broadly, development of a sensor apparatus with a large dynamic range is also desired.
It is an object of the present invention to provide a surface plasmon resonance sensor apparatus in which a surface emitting laser employed as a light source and superior in light directivity, controllability of its polarization plane and suitability for arraying, and a sensor array, such as a CCD array, are arranged on a common substrate, wherein a many-component measurement can be readily performed as well as a single-component measurement by using a light-transmitting medium and a metal thin film provided above the substrate, such that dynamic range can be readily widened.
The objects and advantages of the present invention are achieved by:
A surface plasmon resonance sensor apparatus which comprises:
(a) a common substrate;
(b) a sensor array spaced on said common substrate;
(c) a light-transmitting medium spaced above said common substrate;
(d) a metal thin film formed on said light-transmitting medium; and
(e) a surface emitting laser spaced on said common substrate adapted to emit light through said light-transmitting medium to be reflected from said metal thin film and simultaneously generate a surface plasmon resonance sufficient to change intensity of light reflected from said metal thin film; wherein said surface emitting laser, said metal thin film and said sensor array are positioned to measure the intensity of the light reflected by the metal thin film.
More specifically, a surface plasmon resonance sensor apparatus includes a common substrate, a surface emitting laser, such as a vertical cavity surface emitting laser (VCSEL), arranged on the common substrate, a sensor array, such as a CCD array, arranged on the common substrate, a light-transmitting medium provided above the common substrate, and a metal thin film formed on the light-transmitting medium. When light is emitted from the surface emitting laser is transmitted through the light-transmitting medium and impinges on the metal thin film, a surface plasmon resonance is induced. The surface emitting laser, metal thin film and sensor array are positioned such that the change in intensity of light reflected by the metal thin film, which is caused by the surface plasmon resonance can be measured by the sensor array.
More specifically, the following preferred embodiments may be employed based on the above fundamental invention.
The surface emitting laser and the sensor array are arranged on the common substrate. An optical element is provided above the laser to expand a divergent angle of the light emitted from the surface emitting laser. This optical element can be omitted, if desired. The light-transmitting medium provided with the metal thin film is provided above the substrate. The light-transmitting medium is preferably a resin case filled with air. On an internal surface of the resin case an antireflection film may be formed, and on an external surface, the metal thin film is provided. The respective elements are positioned such that the laser light can be totally reflected at the metal thin film and the light intensity along a direction of the light divergent angle can be measured by the sensor array.
Metal forming the metal thin film is preferably Ag, Au, Cu, Zn, Al or K, as is described in xe2x80x9cSURFACE, Vol. 20 No. 6 (1982) pp. 289-304xe2x80x9d. Ag and Au are particularly preferable. The metal thin film can also be composed of an alloy composition, but an alloy mixing Pd with Au is not good since the surface plasmon disappears when Pd is mixed with Au. Further, the metal thin film can be comprised of, for example, a multi-layer structure in which a very thin film of Cr is formed on the surface of the light-transmitting medium and an Au film or the like is formed on the Cr film to secure a tight contact between the metal thin film and the light-transmitting medium.
As employed herein, the term xe2x80x9cthinxe2x80x9d in the phrase xe2x80x9cmetal thin filmxe2x80x9d refers to a metal film thickness sufficient to totally reflect the light striking it from the light transmitting medium. In general, the thickness of the metal film is from about 100 to 1000 xc3x85, preferably from about 300 to 700 xc3x85 and most preferably about 500 xc3x85 for most metals. A xe2x80x9cvery thinxe2x80x9d film is generally from about 300 to 500 xc3x85 in thickness.
In a specific sensor apparatus, the light radiation point of the surface emitting laser is elliptically shaped. The laser is positioned such that light from the laser enters the metal thin film as p-polarized light. A lens or a hologram device is preferably provided to expand the light beam along its polarization plane, and the total reflection of the light occurs at the metal thin film over a wide incident-angle range. The dependency of the thus-effected surface plasmon resonance on the incident angle is measured by the sensor array, such as a one-dimensional CCD sensor array arranged extending along the above polarization plane on the common substrate.
The number of CCDs in the one-dimensional CCD sensor array is determined by the range of the incident angle of light received by the CCD sensor array and the variation of the surface plasmon resonant angle. Over a hundred (100) CCDs will usually suffice for that purpose, when the incident-angle range is about 10xc2x0 and the resonant-angle variation is about 2xc2x0. Further, where a hologram device is used to expand the laser beam, the beam expansion angle can be freely set by a thin, light element and the above-discussed optical positioning above the surface emitting laser can be readily performed.
In another specific sensor apparatus, a one-dimensional array of parallel-arranged surface emitting lasers and a two-dimensional sensor array are arranged on the common substrate. Light from the surface emitting laser is preferably further expanded in a direction of its polarization plane by the lens or hologram device. However, light beams from adjacently-arranged lasers seldom overlap in a direction perpendicular to the polarization plane since the expansion angle of the laser light in this perpendicular direction is exceedingly small. Thus, undesired crosstalk seldom occurs in the two-dimensional sensor array. Therefore, many components or items can be simultaneously measured when different biosubstances are respectively fixed to different strip portions of the metal thin film on which surface plasmon resonances occur due to the light beams from the respective lasers. Further, when the same biosubstance is fixed to the different strip portions at different concentrations, the sensor construction, in which respective dynamic ranges are varied, can be obtained.
In another specific sensor apparatus, a two-dimensional array of surface emitting lasers and a two-dimensional sensor array are arranged on the common substrate. Light beams from the surface emitting lasers arranged along the direction of the p-polarization plane are preferably transmitted by lenses or hologram devices to enter the interface between the light-transmitting medium and the metal thin film over appropriately-set incident-angle ranges, respectively. When the respective laser incident-angle ranges are continuously set by lenses or hologram devices, a wide incident-angle range, which could not be obtained by a single surface emitting laser, can be covered by the plural lasers. The intensity distribution of totally-reflected light corresponding to this wide incident angle is measured by the sensor array.
Where the complex dielectric constant of the sensitive film fixed to the metal thin film cannot be estimated, the resonant angle causing the surface plasmon resonance cannot be calculated and it is hence difficult to design the apparatus by using a sensor having only a narrow incident-angle range. In such a case, a sensor with a wide incident-angle range is very useful since it can measure the surface plasmon resonance for a wide dielectric constant of the sensitive dielectric film. Further, similarly to the case where the sensor apparatus includes the one-dimensional laser array, when different biosubstances are fixed to the strip portions of the metal thin film, many components or a single component can be measured with a large dynamic range by using a wide incident angle.
When the incident angle of the laser light on the metal thin film covers a large range, the intensity of light to be measured decreases since the radiation angle of light to be received by each sensor element varies broadly. In such a case, an accurate spectrum of the surface plasmon resonance can be obtained when signals detected by sensors, such as CCDs, are corrected.
The sensitive dielectric film to be used in the present invention is formed of a substance which selectively interacts with the measurement object and changes its physicochemical properties, such as its refractive index, thickness and the like. Specifically, the sensitive dielectric film is preferably a polymer film carrying antigen, antibody or the like, a Langmuir-Blodgett film, a polymer film carrying a substance such as a hormone, receptor, polypeptide, nucleic acid, cell, cell membrane, glycoprotein, lipid and pigment, which show an affinity for a specific organic compound, or the like. The measurement object may be a fluid, such as a gas or liquid.
Further, a preferred sensitive dielectric film to be used in the present invention selectively interacts with a nucleic acid to be measured and changes its physicochemical properties, such as its refractive index, thickness and the like. Specifically, a preferred sensitive dielectric film carries probe nucleic acid in which a portion of a base arrangement of a target nucleic acid to be measured is replaced by a complementary base arrangement. More specifically, the sensitive dielectric film contains DNA, RNA, PNA, or the like to act as a probe. In this case, the target nucleic acid is a reaction object. Conversely, the sensitive dielectric film may contain a target nucleic acid. More specifically, the sensitive dielectric film may contain DNA, such as cDNA, or RNA, such as mRNA, tRNA and rRNA. In this case, the probe nucleic acid is a reaction object.
In the thus-fabricated surface plasmon resonance sensor apparatus, the surface emitting laser and the sensor, such as CCD array, are formed on the common substrate and the metal thin film is formed on the light-transmitting medium, such as a resin, provided above the substrate. Thus, all optical elements needed to measure the surface plasmon resonance are disposed on a single chip. Therefore, small-sizing, cost-reduction and excellent productivity of the sensor apparatus can be readily attained. Further, even simultaneous sensing of many components can be achieved by using parallel characteristic (i.e., good directivity) of light emitted from the surface emitting laser and establishing a multi-channel construction. Further, since the surface emitting laser can be readily constructed as an array, a wide incident angle onto the sensor can be attained by combining the arrayed lasers with the sensor array. A many-component sensing sensor apparatus and a sensor apparatus applicable over a wide concentration range can also be achieved.
Features of the surface emitting laser will be described with reference to a typical specific structure.
Surface emitting lasers in a range from blue of about 400 nm to a communication wavelength band of 1.55 xcexcm have been presently developed. They have been studied using a GaN-series on a sapphire substrate, GaAlInP-series, InGaAs-series, GaInNAs-series and GaAlAs-series on a GaAs substrate, GaInAsP-series and GaAlInAs-series on an InP substrate, and other materials. A fundamental structure of a surface emitting laser array is illustrated in FIG. 1.
An epitaxially-grown layer structure 22 with a thickness of about several microns is fabricated on a semiconductor substrate 21, and an active layer 23 is provided in the layer structure 22. Dielectric multi-layer mirrors 24 and 25 with a high reflection factor of over 99% are formed on both surfaces of the layer structure 22. A pixel 26 shows a peripheral shape of the active layer 23, and laser light is emitted perpendicularly to the substrate 21. The reflective layers 24 and 25 are typically formed of multiple layers with a thickness of xcex/4 and different refractive indices, and materials thereof are generally dielectric glasses or epitaxially-grown semiconductors. Examples of the epitaxially-grown mirror are disclosed in xe2x80x9cELECTRONICS LETTERS, 31, p. 560 (1995)xe2x80x9d, wherein AlAs/GaAs multi-layer mirror, active layer and so forth are deposited on an GaAs substrate during a single growth. As is disclosed in xe2x80x9cAPPLIED PHYSICS LETTERS, 66, p. 1030 (1995)xe2x80x9d, a GaAs/AlAs mirror formed on a GaAs substrate is bonded to a laser structure of InGaAsP/InP series grown on an InP substrate, using direct junction. Further, as is disclosed in Japanese Patent Application Laid-Open Nos. 5(1993)-167192 and 6(1994)-237043, the reflective mirror can also be fabricated by epitaxially growing it on a substrate with a hole.
The size of a light emitting portion of the laser device is in a range from 5 xcexcm to 30 xcexcm, and its beam expansion angle is exceedingly small (i.e., its directivity is excellent), compared with those of gas lasers and ordinary semiconductor lasers. Further, light emerging from the laser device can be polarized without using a polarizer, by elliptically shaping the light radiation point of the laser. Moreover, an array of multiple surface emitting lasers can be relatively readily fabricated on a single silicon substrate by using processing techniques.
On the other hand, it is well known that the sensor or photodetector, such as CCD, can be arranged in a one-dimensional or two-dimensional array.
These advantages and others will be more readily understood in connection with the following detailed description of the more preferred embodiments in conjunction with the drawings.